Electronic testing conducted using an automatic probing system requires high-speed performance in order to minimize the number of testers needed to meet manufacturing throughput requirements. Testing of fine-pitch electronic circuit boards is usually done with either a cluster prober or a serial prober. Cluster probers are useful for high density testing, but have the disadvantage of requiring large capital investment for custom probe heads for each product type. Alternatively, serial probers have the advantage permitting probe positioning in a data-driven fashion using programmable map data for selecting successive target positions, and thus can easily be adapted to accommodate testing of a variety of products. However, a disadvantage of serial probers is that they require longer overall test periods for testing highly dense electronic circuit boards compared to cluster probers. Another disadvantage of serial probers is the introduction of test delays which result from the inherent settling time associated with high-speed moving mass systems, e.g., gantry systems, and which are incurred each time the test probe is positioned at a test point target position.
Serial probing systems provide a technique to test fine pitch electronic circuit boards by moving the test probe tip into contact with the circuitry to be tested with a test probe actuator. To take electronic test measurements, a test probe actuator first disengages the test probe from the first test site. Next, the test probe is moved to the next successive test point. After the gantry vibratory motion is stabilized, the test probe is then engaged with the next successive test site by the test probe actuator. Finally, the test measurement is made. This process is repeated until the desired testing of the multiple test sites is completed. The movement of the gantry from one test point to another is monitored and automatically controlled by an x-axis positioning feedback loop and a y-axis positioning feedback loop. Depending on the location of the next successive test point target position, the x-axis gantry positioner and the y-axis gantry positioner can operate either individually, sequentially, or simultaneously to move the gantry to the next test point target position.
In a typical probing process of an electronic device, the gantry is moved to the vicinity of a test point target position. This is followed by a wait period during which the gantry settles at the test point target position. After gantry settling has occurred, the test probe is engaged to the test site by a test probe actuator, a test measurement is made, and the test probe is disengaged from the test site by the test probe actuator. This process is repeated for the testing of successive test sites.
Automatic probing systems typically locate the test probe with respect to test point target positions of a device to be tested. This is usually accomplished with positioning mechanisms which move the test probe over the surface of the device to be tested. After positioning the test probe at the target position, the test probe is engaged to the test site by the test probe actuator to permit a test measurement to be made. After the test measurement has been made, the test probe is disengaged from the test site by the test probe actuator to permit the cycle to be repeated for positioning the test probe to the next successive target position. The test probe can be engaged to the test site either by moving the test probe to the device under test, moving the device under test to the test probe, or moving both the test probe and the device under test toward one another. Similarly, the test probe can be disengaged from the test site either by moving the test probe away from the device under test, by moving the device under test away from the test probe, or moving both the test probe and the device under test away from one another.
A typical positioning mechanism is capable of very large accelerations and is able to move a test probe to a test site relatively quickly. An inherent disadvantage in using these systems is that the positioning mechanism moving mass is large and is difficult to stop and hold to a high degree of accuracy. The larger the working range of the probing system, the larger and heavier the structures required to support the device under test and the test probe, and the longer it takes for the positioning mechanism to settle the moving mass with a high degree of accuracy. Both the test probe positioning bandwidth and the gantry vibratory motion within that positioning bandwidth affect the amount of physical contact and consequent damage to the test probe and to the device under test.
A high performance serial probing system must be responsive, accurate, reliable, and capable of operating without damaging either the device under test or the test probe. To achieve this performance level, large power linear motors or rotary motors are used to move the gantry and the associated test probe with very large positive acceleration, very high transit speed, and very large negative acceleration to successive test point target positions. However, due to the inherent inertia and structural flexibility of the materials comprising this moving mass, the gantry moving mass exhibits vibratory motion as it settles m the test point target position. The settling period necessary to substantially eliminate the gantry vibratory motion can account for as much as 50% of the total test probe move time to successive test point target positions. The gantry vibratory motion is transmitted to the test probe and affects the degree of probing accuracy and the amount of damage which will occur to the device under test and the test probe. To improve probing accuracy and to reduce damage to the device under test and the test probe, it is necessary for the probing system to be idled and to wait for the gantry vibratory motion to decay to within an acceptable range before the test probe and the device being tested can be engaged with one another to permit a test measurement to be made.
For a serial probing system, the test sequence protocol permits the test probe to be positioned m target positions for testing test sites in an arbitrary order. Thus, an optimal test sequence protocol provides for positioning the test probe to the next nearest target position for testing a test site such that the probing sequence comprises many successive short moves and the overall test time is minimized. However, such a serial testing sequence is significantly influenced by the gantry settling time. While this optimal test sequence protocol minimizes the overall serial test time, it exacerbates the affects of the gantry settling time because the settling time accounts for a relatively large percentage of the total time required to perform each serial probing maneuver to successive test point target positions. Typically, as much as fifty percent of the total test probe move time to successive test point target positions is required for the gantry positioner to settle the gantry to within an acceptable distance from the test point target position prior to engaging the test probe to the test site by the test probe actuator.
The productivity and reliability of the serial probing system can be greatly improved if the gantry settling time is shortened or eliminated. One passive technique of achieving reduced gantry settling time would be to use a dashpot element to dampen-out the vibrations. A disadvantage to using the dashpot element form of vibration control in serial probing systems is that the vibratory motion of the servo-driven gantry moving mass has relatively high amplitude and relatively low frequency such that this passive vibratory motion control technique does not effectively suppress the gantry vibratory motion. Therefore, there is a need to be able to actively control gantry vibratory motion to substantially reduce or eliminate test delays resulting from gantry settling wait periods associated with positioning a moving mass.